Available air purification apparatus in the marketplace can be broadly grouped into two principal types: i) removal of unwanted particles through trapping and filtration, and ii) destruction of unwanted particles to eliminated the associated harmful effects or render the harmful microbes (such as bacteria) unviable. Filtration based apparatus have air circulating through an air filtration device (filter) and particles above a certain size are trapped by the filter. Depending on the materials that make up the filter and its construction method, particles with a diameter of a few micrometers or larger can be removed from the circulating air. Some filtration devices incorporate additional apparatus (such as electrostatic precipitators) to introduce charges to the particles to enhance the trapping efficiency. Viruses are generally small enough to pass through these filters and microbes can also be carried by aerosol of sufficiently small size to pass through the filters. Filters trap them but do not destroy them. Some existing air purification systems incorporate ultraviolet (UV) light to destroy trapped microbes. Un-trapped microbes remain in the air.
Ion generators are also provided in the marketplace for the removal of airborne particles. They produce negative electrical charge and when the charge is applied to airborne particles, the airborne particles will fall out and cling to nearby surfaces. Therefore, these devices function only to separate and remove airborne harmful particles but not to destroy them.
There are also ozone (O3) generators in the marketplace for the destruction of airborne microbes. Ozone is not an effective biocide for airborne microbes except at extremely high and unsafe levels, for example, more than 3,000 ppb. As a result, ozone generators cannot destroy airborne microbes or pathogenic microorganisms effectively to achieve any benefits to human health. If these devices accidentally generate excessive levels of ozone, it will be detrimental to health. In fact, there are many articles published giving warnings about use of excessive ozone for air disinfection.
UV has been successfully applied in some applications for disinfection. Research in UV disinfection of airborne microbes demonstrated that a residence time (i.e. the duration that an air stream needs to be radiated with UV) of the order minutes and hours is required to achieve noticeable level of disinfection. This level of efficiency is considered low in practical terms.
There are also articles reported that charged particles (ion clusters) can have disinfection effect. Similar to UV disinfection, the efficiency of this disinfection mechanism is also low and an exposure to the ion clusters of the order hours is typically required.
Plasma is an electrically neutral, ionized gas composed of freely moving ions, electrons, and neutral particles. Plasma is used today for a variety of commercial applications including for air purification and disinfection. Depending on the operation regime, plasma can consist of charged particles (electrons and ions), excited species, free radicals, ozone and UV photons, which are capable of decomposing chemical compounds and destroying microbes. Existing commercially available plasma air purifiers operate either indirectly by using ozone or UV photons generated by plasma contained in a separate device or by charging up the airborne particles in a similar fashion as ion generators operate.
Plasma can be created by electrical means in the form of gaseous discharges whereby a high voltage is applied to a set of electrodes, the anode and the cathode. When the applied voltage is sufficiently high and becomes greater than the breakdown voltage, arcs begin to develop across the electrodes. The threshold for electrical breakdown or arc formation follows the well-known Paschen law, which relates the breakdown voltage to the gap size between the electrodes and the gas pressure.
Breakdown occurs when the applied voltage, or more precisely the local electric field, is sufficiently large for electrons to acquire enough energy to compensate the energy losses due to collisions, excitation and other energy loss processes. The breakdown process begins with presence of some free or residual electrons accelerating towards the anode under the influence of the externally applied electric field. As they accelerate towards the anode, the streaming electrons collide with the gas atoms causing ionization directly by impact or indirectly through photo-ionization. An electron cloud begins to build up and propagates towards the anode together with an ionization or breakdown front ahead of the electron cloud, leaving an ion trail behind, resulting in a plasma channel with an electric dipole opposing the applied electric field. The formation of such streamer, if unrestrained, leads to a rapid increase in charge density, fast growth of an avalanche, and the transformation of the streamer into an arc.
By introducing suitable current limiting or quenching mechanism(s) to prevent the development of major arcs, a quasi-steady state can be established with micro-arcs or filaments (of dimension of the order of 10−4 m) filling up the gap between the electrodes. Traditionally this is achieved by placing dielectric barrier or insulator covering one or both electrodes. Discharge having an insulating or dielectric layer incorporated on one or both of the electrodes is known as dielectric-barrier discharge. The non-conducting property of the dielectric or insulating layer allows charge accumulation on the surface, which produces an opposite electric field to the applied electric field. In addition, the space charge built up next to the dielectric or insulating layer adds to the electron repelling electric field. The opposing electric field cancels the applied electric field and prevents a filament from developing into a major arc and causes a discharge filament to extinguish. Therefore, the low charge mobility on the dielectric leads to self-arresting of the filaments and also limits their lateral extension, thereby allowing multiple filaments to form in close proximity to one another. Furthermore, when coalescence of multiple ionization fronts occurs, the filamentary discharge transforms to a diffuse glow discharge that has spatially more uniform properties. Current quenching can also be achieved by carefully controlling the applied voltage to prevent transition into an arc. It can be created by the use of needle-like electrodes to create a space charge region around the smaller or sharper electrode. It can also be achieved by including non-conducting packing materials in a bed residing between the electrodes.
In a dielectric-barrier discharge operating in the near-atmospheric pressure range, electron energy is typically in the range of 1 to 10 eV and ion energy is close to the ambient gas temperature. Because of the energy disparity between the electron and ion species, these discharges are classified as non-thermal plasma. Typically, the density of the charge particles is much less than the neutral ambient gas and the plasma behavior is dominated by collisional effects. The energy of the electrons can be utilized for exciting atoms and molecules, thereby initiating chemical reactions and/or emission of radiations. The energetic electrons are able to induce the breakdown of some chemical bonds of the molecules, collide with the background molecules resulting in the breakdown of molecular chain, ionization and excitation, and generation of free atoms and radicals such as O, OH or HO2. The radicals can attack hazardous organic molecules and are useful in decomposing pollutants in air. The disassociation of O2 provides the required O to combine with O2 to form ozone. The low energy electrons can attach to neutral atoms or molecules to form negative ions, which can enhance reactions in decomposing pollutants and destruction of microbe. Through collision, electrons can destroy organic compounds including bacteria and virus directly. Emissions, particularly in the UV spectral region, through recombination and relaxation, can initiate photo-physical and photo-chemical process by breaking molecular bonds and hence destroying microbe resulting in disinfection effect.
The harmful contaminants can be broadly grouped into chemical contaminant, volatile organic compounds, bacteria, fungi and viruses; each group is characterized by the amount and complexity of the constituent molecules or radicals. Plasma properties have to be optimized in order to destroy and/or to prevent the growth of these harmful contaminants. One major requirement is to ensure that these harmful contaminants have an adequate residence time within the reactor device while supporting a sufficiently high flow rate for practical applications.
While plasma can be utilized to reduce the harmful particles, the generation of plasma also creates by-product gas which can be hazardous. Typical examples of bi-product gas are ozone and nitrogen dioxide (NO2).
Therefore it would be desirable to provide an apparatus to generate plasma for indoor air purification and disinfection which adequate residence time and effective plasma power deposition to achieve efficient destruction of pollutants while minimizing the generation unwanted bi-product gases.